An APN, short for Access Point Name, is an identifier for a Packet Data Network (PDN). In other words, the APN identifies the PDN that a user of a wireless device wants to communicate with. The APN may also be used to define a type of service that is provided by the APN. The APN is used in both the General Packet Radio Service (GPRS) and the Evolved Packet System (EPS).
Very often, PDN connections or Packet Data Protocol (PDP) contexts for a certain APN have different needs with regards to functionality that is required in the PDN-GateWay/Gateway General packet radio service Support Node (PDN-GW/GGSN) or in any other nodes that may be located on the Gi/SGi interface e.g. a node implementing a Traffic Detection Function (TDF). This may be due to the fact that many network operators may want to provide differentiation in their service offerings to the end users or they may for other reasons want to treat the PDN-connections or PDP contexts differently based on a number of different factors such as location of the wireless device, time-of-day/day-of-week, accumulated data usage for some time, wireless device type etc. For example for the ordinary Internet APN:                Some users may have pre-paid subscriptions that require Online Charging to be enabled, while other users may be post-paid and do not have the need for Online Charging. However Online Charging is a GW function that is typically enabled/disabled per APN, although the function as such is very resource consuming for the GW.        Some users, i.e. children, may require content-filtering/parental control to be enabled for their subscriptions. Typically this is a very low percentage of all users (e.g. <5%).        Some users may have flat-rate subscriptions that are independent of which service that is used, while other users may e.g. only be allowed to access certain services for free while others are paid for per megabyte. For the latter, service awareness is required in the PDN-GW/GGSN or TDF and therefore Packet Inspection and Service Classification (PISC) needs to be enabled for the entire APN. PISC is a very resource consuming function for the PDN-GW/GGSN and the TDF.        Also, within the domain of packet inspection there are differences. PISC may be used for policy and charging, which requires one set of filter configuration. PISC may also be used e.g. to obtain service usage statistics for the purpose of analytics, for example to get a list of the most popular domains accessed (Uniform Resource Locators (URLs)), and this may require a different (more granular) filter configuration.        
The above listed functions are all examples of functionality that is typically configured per APN in the PDN-GW/GGSN and the TDF. This means that in case some users that may access this APN may use the function, then the function must be enabled.
The Gi and SGi interfaces mentioned above will now be briefly explained. The SGi interface connects the PDN-GW to an external network (e.g. a PDN) for the EPS e.g. for Long Term Evolution (LTE) access. The Gi interface connects the GGSN to an external network (e.g. PDN) for GPRS access.
PCC Architecture in 3GPP
The architecture that supports Policy and Charging Control (PCC) functionality is depicted in FIG. 1. This figure has been taken from TS 23.203 (V.12.2.0) that specifies the PCC functionality for Evolved Third Generation Partnership Project (3GPP) Packet Switched domain, including both 3GPP accesses (GERAN/UTRAN/E-UTRAN) and Non-3GPP accesses. GERAN is short for GSM EDGE Radio Access Network, GSM is short for Global System for Mobile communications and EDGE is short for Enhanced Data rates for GSM Evolution. UTRAN is short for UMTS Terrestrial Radio Access Network, UMTS is short for Universal Mobile Telecommunications System and E-UTRAN is short for Evolved-UTRAN.
The PCRF 101 is a functional element that encompasses policy control decision and flow based charging control functionalities. The PCRF 101 provides network control regarding the service data flow detection, gating, Quality of Service (QoS) and flow based charging (except credit management) towards a Policy and Charging Enforcement Function (PCEF) 103. The PCRF 101 receives session and media related information from an Application Function (AF) 105 and informs the AF 105 of traffic plane events.
The PCRF 101 shall provision PCC rules to the PCEF 103 via the Gx reference point 106. The PCRF 101 shall provision Application Detection and Control (ADC) rules to a TDF 108 via the Sd reference point 109.
The PCRF 101 shall inform the PCEF 103 and TDF 108 through the use of PCC and ADC rules on the treatment of each service data flow that is under PCC/ADC control, in accordance with the PCRF 101 policy decision(s).
The AF 105 is an element offering applications in which service is delivered in a different layer (i.e. transport layer) from the one where the service has requested the control of Internet Protocol (IP) bearer resources according to what has been negotiated (i.e. signaling layer). One example of an AF 105 is the Proxy-Call Session Control Function (P-CSCF) of the IP Multimedia (IM) CN subsystem. The AF 105 shall communicate with the PCRF 101 to transfer dynamic session information (i.e. description of the media to be delivered in the transport layer). This communication is performed using the Rx interface 110.
The PCEF 103 encompasses service data flow detection (based on the filter definitions included in the PCC and ADC rules), as well as online and offline charging interactions (not described here) and policy enforcement. Since the PCEF 103 is the entity handling the bearers it is where the QoS is being enforced for the bearer according to the QoS information coming from the PCRF 101. This functional entity is located at the gateway (e.g. GGSN in the GPRS case, and PDN-GW in the EPS case). For the cases where there is Proxy Mobile IPv6 (PMIP) instead of GPRS Tunneling Protocol (GTP) protocol between a Bearer Binding and Event Reporting Function (BBERF) 113 and the PCEF 103, the bearer control is done in the BBERF 113 instead. Gxx 115 is the interface between the BBERF 113 and the PCRF 101.
The Sp interface 118 lies between the PCRF 101 and a Subscription Profile Repository (SPR) 120. The Gy reference point 123 resides between an Online Charging System (OCS) 125 and the PCEF 103. The Gy reference point 123 allows online credit control for service data flow based charging. The Gz reference point 128 resides between the PCEF 103 and an Offline Charging System (OFCS) 130. The Gz reference point 128 enables transport of service data flow based offline charging information. The Sy reference point 133 resides between the PCRF 101 and the OCS 125. The Sy reference point 133 enables transfer of policy counter status information relating to subscriber spending from the OCS 125 to the PCRF 101. The Gyn reference point 135 resides between the OCS 125 and the TDF 108, and allows online credit control for charging in case of ADC rules based charging in the TDF 108. The Gzn reference point 138 resides between the TDF 108 and the OFCS 130 and enables transport of offline charging information in case of ADC rule based charging in TDF 108.
APN Selection in the MME/SGSN
The APN may be provided from the wireless device to the Mobility Management Entity-Serving GPRS Support Node (MME-SGSN) during an Initial Attach request or a PDN-connectivity Request for LTE access and in the PDP-context activation request for Wideband Code Division Multiple Access (WCDMA) and GSM access. A default APN may also be provided from a Home Subscriber Server (HSS) (for WCDMA access) or a Home Location Register (HLR) (for GSM access).
A Default APN may be defined as the APN which is marked as default in the subscription data and used during the Attach procedure and the PDN connectivity procedure requested by the wireless device when no APN is provided by the wireless device.
The term MME-SGSN may refer to a MME, a SGSN or a combined MME and SGSN node.
If one of the PDN subscription contexts provided by the HSS comprises a wild card APN, a PDN connection with dynamic address allocation may be established towards any APN requested by the wireless device. A wild card APN does not specify a specific APN.
If the wireless device provides an APN, this APN shall be employed for default bearer activation.
The selected APN is used, typically via a Domain Name Server (DNS)-lookup, to identify which PDN-GW/GGSN that the PDN-connection shall be assigned to. It will also in that PDN-GW/GGSN, based on local configuration, be used to determine what GW-functions that will be assigned to the PDN-connection there. For a certain APN multiple supporting PDN-GW/GGSNs may be defined enabling a scalability and redundancy.
Many functions in the PDN-GW/GGSN and in the TDF are enabled and disabled per APN. Due to the fact that functional requirements for different PDN-connections/PDP-contexts for the same APN are not homogenous it is hard to optimize the performance of the APN and the nodes.